Method and composition for decontamination of stainless steel surfaces



States ABSTRACT OF THE DISCLOSURE The tenacious magnetite filmcontaining included radioactive contaminants which forms on stainlesssteel heat exchange surfaces of nuclear reactors is removed by a twostep treatment. The first step utilizes a hot, alkaline, aqueoussolution of potassium permanganate. The second utilizes a hot aqueoussolution of oxalic acid, dibasic ammonium citrate, ferric sulfate ornitrate and diethyl thiourea.

CONTRACTUAL ORIGIN OF THE INVENTION This invention was made in thecourse of or under a contract with the United States Atomic EnergyCommission.

Introduction The invention relates to a method of and a composition forthe removal of radioactive contamination from metal surfaces. It iseffective for removing the extremely tenacious radioactive film whichforms on stainless steel surfaces on long exposure to hot Watercontaining radioisotopes. It is also effective for removing the lesstenacious radioactive films formed on carbon steel, zircaloy, brass,bronze and other metals. At the same time there is no objectionablecorrosion to any of these metals.

The principal object of the invention is the decontamination of nuclearreactor cooling systems composed primarily of stainless steel. Theradioactive corrosion films characteristic of recirculating water-coolednuclear reactors must be periodically removed to permit contactmaintenance and continuity of operation. Before these systems can becleaned, procedures and processes must be developed which are suitablefor use in the system. These processes must meet certain criteria whichare discussed in the following paragraphs.

A decontamination reagent must dissolve the radioactive corrosion filmand remove it from the system. Large nuclear plants are no exception toother types of plants; they are built with inherent low velocity areasand other traps where particulate material will settle out. Theparticulates in these areas must be dissolved for successful removal.Therefore, a successful decontamination reagent must dissolve as well asremove the radioactive corrosion film because if it is only removed fromthe piping surfaces and not dissolved, it will settle out in the lowvelocity areas and traps.

A successful decontamination reagent must remove and dissolveradioactive films formed during extended periods of continuous operation(up to at least two or three years). From the limited data available inthe AEC literature, it is probable that cleaning will not be requiredoftener unless under unusual circumstances or demands. It is desirous,in power reactor operation, to keep the down time at a minimum andpossibly reach five years of continuous operation before cleaning.Therefore, the removal of these long term films is an essentialrequirement.

A successful decontamination reagent must not be excessively corrosiveto the materials of construction in the primary system. The reactorsystem will be constructed of primarily one or two materials, but therewill be small quantities of many other types of materials in the system.These lesser materials will be found in such places as valves andsampling devices. All of these materials must be compatible with thereagent if they are in either direct or indirect contact with thesolution, this is particularly true if the components are in a criticallocation, e.g., a drain valve plug, seat or stem.

The film which forms on stainless steel surfaces in the cooling systemsin water cooled nuclear reactors appears to be primarily magnetite mixedwith oxides of nickel and chromium. Radioactive ions present in thewater are included in or adsorbed on this film so that they cannot beremoved without removing the film itself. It is known in the art todecontaminate stainless steel nuclear reactor cooling systems bytreatment with alkaline potassium permanganate followed by oxalic acid.However, previously known oxalic acid containing solutions have tendedto deposit a secondary film on the surfaces. Sometimes the secondaryfilm takes the form of a precipitate settling out in the low velocityregions and dead legs of the cooling system. This secondary filmcontains a considerable amount of radioactivity and its formationpartially nullifies the decontamination.

General Description of Invention In my procedure I first treat thesystem with the conventional alkaline potassium permanganate. Thisserves primarily for the removal of chromium and conditions the film sothat it is dissolved in the acid medium. In the subsequent treatment Iutilize a novel oxalic acid-containing aqueous solution which preventsthe formation of the secondary film referred to above. This solution hasthe following composition:

G./l. Oxalic acid 20-30 Dibasic ammonium citrate 40-60 Ferric sulfate ornitrate 1.7-2.4 Diethyl thiourea 0.81.2

The ratio of oxalic acid to dibasic ammonium citrate should not begreater than /2, otherwise secondary film formation may occur.

The preferred composition at present is as follows:

G./l. Oxalic acid 25 Dibasic ammonium citrate 50 Ferric sulfate 2Diethyl thiourea 1 This solution, which is designated RDW-3, was used inthe experimental tests and the reactor decontamination subsequentlydescribed. If significant surfaces of carbon steel or of stainless steelof the 400 series are present, it is preferable to use 1.2 g./l. ofdiethyl thiourea in the RDW-3 solution.

Detailed Description The following examples are based on decontaminationof the primary cooling system of the Plutonium Recycle Test Reactorlocated near Richland, Wash. It is a heavy water moderated and cooledreactor of the pressure tube, pressurized water type. A completedescription of the PRTR is given in US. Atomic Energy Commission ReportHW-61236, Plutonium Recycle Test Reactor Final Safeguards Analysis.

Except for the zircaloy-Z pressure tubes, in which the fuel elements aremounted, the primary cooling system is composed principally of type 304stainless steel.

3 EXAMPLE I Experimental tests Samples were cut from various jumpers inthe primary cooling system of the PRTR. These jumpers had been subjectedto filming by the circulating coolant for various periods ranging from 6to 32 months.

The samples were treated at l105 C. with 3% KMnO solution containingvarying concentrations of NaOH and then at 80 C. with the RDW-3solution. Results are shown in Table I.

TABLE I.DECONTAMINATION RESULTS FOR THE RDW-3 SOLUTION Jumper NumberMonths of Filming 6 6 12 16 20 22 22 27 27 29 32 32 a IA=Initialactivity mrad/hr. FA=Final activity mrad/hr.

No secondary films or precipitates were formed. It will be noted thatthe treatment was particularly elfective on those samples exhibitinghigh initial activities.

Corrosion tests were then conducted on samples of various metals. Onecorrosion test was conducted with the RDW-3 solution at 80 C. for 120hours; the total corrosion incurred by the carbon steel samples exposed3 was 0.22 mil and by the stainless steel samples was 0.01 mil. A paleyellow film was deposited on the carbon steel samples when the test wasterminated; the solution was a clear yellow color with no precipitatespresent.

Another corrosion test was conducted for a 24-hour period at 82 C.;numerous other types of materials were charged during this test. Thecorrosion data obtained are presented in Table II. The corrosion attackon the carbon steels and the 440-A and 440-C stainless steels increasedwith increased exposure while the attack on the remaining alloys did notincrease after the initial attack. The attack to all alloys was verylow; the most attack (0.2 mil) was on Stellited A212 carbon steel.Pitting did not occur on any of the carbon steel alloys, but slightpitting up to 6 mils diameter occurred on the 400 series stainlesssteels.

There was no deposition with the RDW-3 solution in contrast with otheroxalic acid containing solutions. The solution was a clear yellow, withno precipitates.

Dull grey passive oxides formed on the carbon steel and 400 seriesstainless steels; these oxides proved to be resistant to subsequentrusting when the samples were rinsed in warm tap water.

Tests were also conducted on zircaloy-Z. No corrosion was apparent onthe samples of this alloy.

s DF=Decontamination Factor=IA/FA.

flushed with water until the latter had attained a pH between 10 and 11.

The RDW-B solution was then circulated through the system for threehours. It was maintained at C. for one hour. The system was then flushedwith deionized water until the latter attained a resistivity of 10,000ohm centimeters. The results are given in Table III.

TABLE IIL-DECONTAMINATION FACTORS OBTAINED ON PRTR. PRIMARY SYSTEMActivity Levels (mR/hr.) Decontamination System Location Before AfterFactor Inlet header, injection pumps- 60 5 12 Outlet header, injectionpumps 100 5 20 Outlet line, DT-2 Tank.- 5 l8 Inlet line, DT-Z Tank 5 20Pressurizer, outlet piping- 190 5 38 Inlet, primary pump, 2 220 5 44Inlet, primary pump, 1 210 5 42 Inlet, lower ring header- 300 10 30Bypass, HX-l and pressurizer 100 5 20 Piping, HX-l to pressurizer. 5 22Volute, primary pump, 2.- 5 30 Volute, primary pump, 3 120 5 24 Inlet toDegasser 100 5 20 Bottom of HX-5 header 190 5 38 HX-l general backgroundon enclosure grating 25 6 4 HX-l Secondary side:

6 inside manhole 1, 200 5 240 1 from the tubes 1, 800 5 360 Maximumreading on tubes 4, 500 25 In the above table the abbreviations usedhave the following meanings: DT-2=storage tank; HX-1=primary heatexchanger; HX-5= auxiliagiy3 heat exchanger. There are three primarypumps, numbered 1, 2 an TABLE II.RESULTS OF RDW-3 CORROSION EVALUATIONPenetration, mils 1 Sample Appearance Alloy 8 hours 16 hours 24 hours 16hours 24 hours Haynes 25 0.015 0. 016 0.017 Shiny Shiny. 304 SS,stressed 0. 0061 0. 006 0. 0067 -do D0. 304 SS, sensitized 0.0051 0.0065 0. 0069 do D0. 316 SS 0. 0060 0. 0042 0. 0058 -(10 D0. 17-4 PH SS0.024 0.015 0.021 .d0 Do. 416 SS 0. 074 0. 086 0. 085 Dull grey Dlllgrey-5 pits to 6 mils 18.111 91'. 440-A SS 0.063 0. 069 0. 11 Dull grey,slight attack at Dull grey, slight attack at Dull grey, surfaceroughencrevice area. crevice area. ing. 440-0 SS 0. 094 0. l3 0. 198 Afew scattered 2-3 mil Dull grey, no pits Surface very rough, a few 3diameter pits. mil diameter pits. Stellited A212 CS 3 0. 046 0.10 Dullgrey Dull grey Dull grey. A212 CS Welded to 304 SS- 0. 024 0. 029 45 ddo D0. CS 0. 046 0. 054 do. Do. 0. 044 0. 078 do Do.

3 GS carbon steel.

The zirca1oy-2 pressure tubes were coated with an iron film containingcontaminants. This film was completely removed, leaving a brightsurface. To the best of my knowledge this represents the most successfulnuclear reactor decontamination ever recorded.

While I have described specific embodiments of my process, it will beunderstood that various changes are possible. I, therefore, wish myinvention to be limited only by the scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of removing a tenacious magnetite film containing includedradioactive contaminants from a nuclear reactor cooling system composedprimarily of stainless steel, which system has been subject to extendedcirculation of hot water during recator operation, which methodcomprises:

(a) circulating through said system of hot alkaline,

aqueous, potassium permanganate solution and thereafter (b) circulatingthrough said system a hot aqueous solution of the following composition:

Oxalic acid 20-30 Dibasic ammonium citrate 40-60 Feric sulfate ornitrate 1.7-2.4

Diethyl thiourea 0.8-1.2

6 the ratio of oxalic acid to dibasic ammonium citrate being not greaterthan /2.

2. A method as defined in claim 1 wherein the solution employed in Stepa. contains 3% potassium permanganate and 10% sodium hydroxide.

3. A method as defined in claim 2 wherein the solution employed in Stepb. has the following composition:

LEON D. ROSDOL, Primary Examiner W. SCHULZ, Assistant Examiner US. Cl.X.R.

